Exhaust gas sensor and process for producing same

ABSTRACT

A combustion gas sensor and material therefore responsive to changes in the composition of ambient atmospheres wherein the sensing element includes SnO 2  crystals having a mean d of approximately 500-3200 Å and having a surface area to mass ratio S of 1-8 m 2  /g a process for making same. Preferably, the standard deviation of crystal size distribution is 0.2d.

This is a division, of application Ser. No. 482,239, filed Apr. 5, 1983,now U.S. Pat. No. 4,459,577.

BACKGROUND OF THE INVENTION

The present invention relates to improvements in combustion gas sensorshaving SnO₂ incorporated therein and a process for producing same. Moreparticularly the invention relates to exhaust gas sensors wherein use ismade of the fact that the electrical conductivity of SnO₂, varies withthe composition of ambient atmospheres, i.e. O₂, CO, and H₂concentrations.

DESCRIPTION OF THE PRIOR ART

Sensors responsive to changes in the composition of ambient atmosphereshave various applications and have been found particularly useful indevices for regulating the air/fuel ratios in combustion type devices,eg. furnaces, space heaters, internal combustion engines, etc., tooptimize fuel efficiency, reduce pollution, and the like.

For example, it is well known that the operation of an internalcombustion engine produces substantial quantities of deleterious gaseousby-products. The principal pollutants so produced are hydrocarbons,carbon monoxide and various oxides of nitrogen. Extensive investigationshave led to the discovery that the use of a catalytic converter withinthe exhaust system of an internal combustion engine provides a practicaltechnique for reducing the emission of the deleterious gaseousby-products.

A catalytic exhaust gas treatment device, or `converter`, which iscapable of converting the principal pollutants into water, carbondioxide, and gaseous nitrogen is often referred to as a "three-waycatalyst". For the three-way catalyst devices to be most effective, theexhaust gases introduced into the converter for treatment must be theproduct of the combustion of a substantially stoichiometric air/fuelratio. Assuming that λ is the stoichiometric air/fuel ratio the regionwhere the converter is most effective extends from about 0.99λ to about1.01λ.

In view of the narrowness of the region wherein the catalytic converteris most effective, it has been determined that the associated internalcombustion engine must be operated with a combustible mixture as closeas possible to stoichiometric equivalence.

To assure continuous or substantially continuous operation at theoptimum air/fuel ratio, it has been proposed to employ sensorsresponsive to the chemistry of the exhaust gases. One known type ofexhaust gas sensor employs a ceramic material which demonstrates apredictable electrical resistance change when the composition of theexhaust gas changes. An example of such a material is titania (titaniumdioxide having a general formula TiO₂). Such sensors may be fabricatedin accordance with the teachings of U.S. Pat. No. 3,886,785 issued toStadler et al., titled "Gas Sensor and Method of Manufacture".

Such exhaust gas sensors are also useful for preventing incompletecombustion. For example, air or space heaters, furnaces and likecombustion type devices may be regulated to avoid incomplete combustionby detecting the composition of the exhaust gas from the device with anexhaust gas sensor and associated means for altering the air/fuelmixture accordingly.

Published Unexamined Japanese Patent Application No. 55099/1978corresponding U.S. Pat. No. 4,194,994; and West German PatentApplication No. 2,648,373 disclose an exhaust gas sensor employing SnO₂(stannic oxide). These publications disclose an exhaust gas sensor,which is prepared by admixing a small amount of Nb₂ O₅ or MgO with SnO₂and calcining the mixture at about 650° to 850° C. These sensors exhibitmarked variations in electrical conductivity in response to variationsoxygen partial pressure in ambient atmospheres at about 600° C.

Published Unexamined Japanese Patent Application No. 19592/1976 andcorresponding U.S. Pat. No. 4,033,169 and West German Patent ApplicationNo. 2,535,500 disclose that the sensitivity of SnO₂ to hydrocarbonsdecreases greatly at temperatures greater than about 300° C.

Exhaust gas sensors must withstand both oxidizing atmospheres andreducing atmospheres, while they are not infrequently heated totemperatures of about 900° C. This means that the exhaust gas sensormust be resistant to various atmospheres at high temperatures.

Furthermore, the exhaust sensors must be rapidly responsive and highlysensitive to changes in atmospheres.

Because exhaust gas sensors are heated by exhaust gases, they oftenexperience temperatures that vary over the range of about 400° to 900°C. in accordance with variations in the combustion conditions concernedand the design of the particular device. It is therefore preferred thatexhaust gas sensors be operable over a wide temperature range and thatthe resistivity of sensors be less dependent on the temperature toassure higher detection accuracy.

It is also desirable that exhaust gas sensors transmit large inputsignals to the circuits connected thereto, consequently, it is desirableto reduce the overall resistivity of the sensor itself.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an exhaust gas sensorwhich is stable in various atmospheres at high temperatures.

Another object of the invention is to provide an exhaust gas sensorhaving high sensitivity and rapid responsiveness to changes ofatmospheres.

Another object of the invention is to provide an exhaust gas sensoroperable a wide range of operating temperatures.

Another object of the invention is to provide an exhaust gas sensorhaving a low resistance that is not greatly affected by temperaturechanges.

Still another object of the invention is to provide a process forproducing exhaust gas sensors which fulfill the foregoing objects.

It has been discovered that sensors responsive to changes in the O₂, CO,and/or H₂ concentrations in ambient atmospheres may be improved, inaccordance with the foregoing objects, by controlling the surface areato mass ratio S, hereinafter defined, and mean crystal size d,hereinafter defined, in sensing elements containing at least some metaloxide particularly SnO₂.

A preferred exhaust gas sensor in accordance with the present inventionhas incorporated therein a sensing element composed of SnO₂ fordetecting changes in the composition of ambient atmospheres and ischaracterized in that the SnO₂ sensing element has a 1 to 8 m² /g insurface area ratio S and 3200 to 500 Å mean crystal size d. An even morepreferable range for S is between about 1.4 and 2.5. With the exhaustgas sensor of the present invention, it is especially preferable thatthe standard deviation of crystal size distribution of the SnO₂ be atleast 0.2d based on d³ N(d) wherein d is the size of each crystal andN(d) is the distribution of crystal sizes.

The process of the present invention for producing sensors comprisesheating tin or a compound thereof in an non-reducing atmosphere toprepare SnO₂ having a surface area S of 1 to 8 m² /g and a mean crystalsize d of 3200 to 500 A. SnO₂ with a wide distribution of crystal sizescan be prepared from a stannic acid sol which contains anion impurities,such as Cl⁻, Br⁻, I⁻, F⁻, NO₃ ⁻ or SO₄ ²⁻, by heating the sol in anon-reducing atmosphere to obtain as an intermediate product SnO₂containing such anion impurities in an amount of 0.14 to 0.60mmol/g.SnO₂, and heating the product again to approximately 1000° to1400° C.

The surface area ratio S herein referred to is a value measured by theB.E.T. method while the mean crystal size d is the mean diameter ofcrystals in the direction of D(1,1,0) plane.

The surface area ratio S and the mean crystal size d are thus specifiedfor the following reasons.

There are three kinds of SnO₂ materials for exhaust gas sensors.

The first, (hereinafter referred to as "SnO₂ (A)") is SnO₂ having asurface area of at least 10 m² /g and a mean crystal size of up to 400Å. SnO₂ (A), which is used for detecting combustible gases in anatmosphere, is suited for detecting small amounts of combustible gasesin the presence of large quantities of oxygen. The characteristics ofexhaust gas sensors wherein SnO₂ (A) is used alter when the sensor isexposed to a hot reducing atmosphere. If these sensors are exposed toreducing atmospheres at about 900° C. for a prolonged period of time andthen to oxidizing atmospheres, their resistivity is lowered permanentlyand even if heated in the oxidizing atmosphere for a long period oftime, the material fails to return to its initial resistivity. When SnO₂(A) is exposed to a reducing atmosphere for shorter periods of time, theresistivity thereof suffers hysteresis, such that the material exhibitsa temporarily reduced resistivity when subsequently exposed to anoxidizing atmosphere. Moreover, this material demonstrates reduced ratesof response to changes from reducing atmospheres to the oxidizingatmospheres.

The second, (hereinafter referred to as "SnO₂ (C)") is SnO₂ having asurface area of up to 0.7 m² /g and a mean crystal size of at least 5000Å. Since preparation of SnO₂ (C) involves accelerated crystal growthresulting in decreased proportions of defects in the crystals, thematerial is higher in resistivity and in temperature coefficient ofresistance. SnO₂ (C) also has reduced surface activity and is thereforelow in sensitivity and in responsiveness. Although calcined at a hightemperature, SnO₂ (C) is prone to crystal growth and has low resistanceto heat. For example, when heated at 900° C., the material exhibits anirreversibly altered resistivity in an oxidizing atmosphere. Stated morespecifically, heating in an oxidizing atmosphere imparts an increasedresistivity to the material, while heating in a reducing atmospheregives a reduced resistivity to the material.

In contrast, the tin dioxide of the present invention (sometimesreferred to herein as "SnO₂ (B)") which has an S of 1 to 8 m² /g and a dof 3200 to 500 Å exhibits outstanding characteristics as a material forexhaust gas sensors. SnO₂ (B) is stable under various atmospheres athigh temperatures, free from substantial variations in its resistivitywith the lapse of time, and rapidly responds to the change from areducing atmospheres to oxidizing atmospheres. Additionally, SnO₂ (B) ishighly sensitive and rapidly responsive over a wide temperature rangeand has a low resistivity which is less dependent on temperature that ofthe above noted SnO₂ types.

These differences in characteristics appear attributable to thefollowing. When subjected to a hot reducing atmosphere, SnO₂ (A) whereinthe crystals have not grown fully, may be reduced to the interior of itslattice. When the lattice is reduced, sintering proceeds through thereduced portion, resulting in decreased resistance at the interfacebetween the crystals. When the reduced material is placed in anoxidizing atmosphere, hysteresis that is delayed response will resultbecause the interior of the crystals must be reoxidized. SnO₂ (C) issusceptible to sintering and is therefore unable to retain itselectrical properties after exposure to high temperatures.

For the reasons given above, the SnO₂ (B) to be used in this inventionis adapted to have an S value of about 1 to 8 m² /g and a d of 3200 to500 Å.

Preferably, the standard deviation σ(d) of the crystal size distributionof the SnO₂ is at least 0.2d based on d³ N(d) wherein d is the meancrystal size, d is the size of each crystal and N(d) is the distributionof crystal sizes. Further d³, i.e., the volume of each crystal, is takenas the weight factor of the distribution because what appears to be ofsignificance is the volume or weight ratio of SnO₂ of crystal sizesrelative to the whole SnO₂, rather than the mere number of particleshaving the crystal sizes.

With SnO₂ prepared by a usual method, for example, by thermallydecomposing Sn(CO₂) in an oxidizing atmosphere, or by repeatedly washinga stannic acid sol with water to fully remove anion impurities therefromand calcining the sol, the standard deviation of the crystal sizes isabout 0.1d.

However, when a stannic acid sol, containing substantial quantities ofanion impurities is calcined the standard deviation of crystal sizedistribution of the resulting SnO₂ may be as great as at least 0.2d.When the electrical characteristics of materials having varyingdistributions of crystal sizes are compared, it is noted that the widertheir distribution, the higher their sensitivity and responsiveness atlow temperatures. Accordingly, it is desirable to use SnO₂ with a widercrystal size distribution to provide a sensors which perform better overa wider range of temperatures.

The upper limit for crystal size distribution may generally be greaterthan 0.6d and is certainly greater than 0.45.

SnO₂ having a large distribution of crystal sizes can be prepared fromSnO₂ having lesser crystal size distributions containing substantialquantities of anion impurities, such as Cl⁻, Br⁻, I⁻, F⁻, NO₃ ⁻ or SO₄²⁻, by heating the material at 1000° to 1400° C. During the heattreatment sometimes referred to as calcining, the anion impuritiescontained in the material appear to cause a widened distribution ofcrystal sizes. SnO₂ containing a substantial quantity of anionimpurities may be obtained by thermally decomposing stannic acidcontaining at least 0.11 mmol/g.Sn(OH)₄ of anion impurities to SnO₂. Itis critical that the resulting SnO₂ intermediate material contain 0.14to 0.60 mmol/g.SnO₂ of anion impurities. With less than 0.14 mmol/g.SnO₂or more than 0.60 mmol/g.SnO₂ of impurities present, the SnO₂ eventuallyobtained will have a distribution of crystal sizes less than thatpreferred in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the configuration of an exhaust gas sensorembodying the invention.

FIG. 2 is a characteristics diagram showing the relation of thesecondary calcination temperature T₂ (hereinafter defined) to thesurface area ratio S as well as with the mean crystal size d, asdetermined with samples prepared by reacting metal Sn with nitric acidand subjecting the resulting stannic acid to primary calcination at 600°C.

FIG. 3 is a characteristics diagram showing the relation of thesecondary calcination temperature T₂ with the surface area S as well aswith the mean crystal size d, as determined with samples prepared byhydrolyzing an aqueous solution of SnCl₄ with ammonia to obtain astannic acid sol containing a large amount of Cl⁻ ions and subjectingthe sol to primary calcination at 600° C.

FIG. 4 is a diagram showing characteristics of samples prepared byneutralizing SnCl₄ with ammonia to obtain stannic acid containing alarge amount of Cl⁻ ions, subjecting the acid to primary calcination atvarying temperatures T₁ and further subjecting the products to secondarycalcination at 1200° C.

FIGS. 5 and 6 are electron photomicrographs showing SnO₂ samples (notpulverized) prepared from a stannic acid sol containing Cl⁻ ions.

FIG. 7 is a diagram showing the characteristics of a sample prepared bythermally decomposing Sn(CO₂)₂ in oxygen.

FIG. 8 is a diagram showing the characteristics of a sample prepared byhydrolyzing an aqueous solution of SnCl₄ with ammonia to obtain astannic acid sol, washing the sol with water to completely remove theCl⁻ ions and thermally decomposing the sol in air.

FIGS. 9 to 11 are characteristics diagrams showing the resistivities ofexhaust gas sensors in an oxidizing atmosphere (solid lines) and in areducing atmosphere (broken lines).

FIGS. 12 and 13 are characteristics diagrams showing the responsivenessof exhaust gas sensors to changes of atmospheres.

FIG. 14 is a characteristics diagram showing variations in theresistivity of exhaust gas sensors at 900° C. in an oxidizingatmosphere.

DESCRIPTION OF PREFERRED EMBODIMENTS EXAMPLE A

A 100 g quantity of metal Sn is added to 800 ml of 6N nitric acid, andthe mixture is reacted overnight at about 50° C., giving a stannic acidsol containing a large amount of NO₃ ⁻ ions. The reaction mixture isfiltered to remove the unreacted residue, the filtrate is placed into arotary kiln, and the kiln is continuously heated at 300° C. untildisappearance of NO₂ evolution. The resulting stannic acid gel is heatedin air at 600° C. for 2 hours (primary calcination) to obtain SnO₂ stillcontaining NO₃ ⁻ ions. The primary calcined SnO₂ is then pulverized wetin a ball mill for 6 hours, molded into a pellet-shaped ceramic memberhaving a pair of Pt-Rh alloy wires embedded therein, and then heated inair at 1050° to 1350° C. for 2 hours (secondary calcination). Theceramic member obtained is mounted on a support member to fabricate anexhaust gas sensor shown in FIG. 1. With reference to this drawing,indicated at 1 is the support member which is made of a base plate ofalumina or like ceramic. The support member 1 is formed with a hole 2close to its forward end. The ceramic element 3, i.e., the SnO₂ pellet,is accommodated in the hole 2. The SnO₂ ceramic element 3 may furtherincorporate therein various other additives, such as Al₂ O₃ serving asan aggregate, amorphous silica as a binder for SnO₂, and/or Pt, Rh orthe like as sensitivity enhancing agents.

It will be appreciated that the additions of binders sensitizing agentsand the like as contemplated by the present invention as well as,molding conditions, e.g. pressure, may have some effect on S butexperience has shown that these effects are small and insignificant forpurposes of this invention seldom exceeding 10%. A pair of Pt-Rh alloywires embedded in the ceramic member 3 to serve as output electrodes 4which conduct current from the ceramic element 3 and may also supportthe ceramic member 3 within hole 2. The wires comprising electrodes 4extend thru a pair of elongated grooves 5 formed in the support member 1to retain the ceramic element 3 in the hole 2. Preferably electrodes 4are held in grooves 5 by an inorganic adhesive 6. The base ends of thealloy wire electrodes 4 are connected to metal pins 7 for electricallyconnecting the electrodes to an external circuit. A pair of holes 8 maybe provided in support member 1 for attaching the sensor to a combustionchamber, or the like, by any suitable means such as bolts and nuts.

The SnO₂ may be subjected to secondary calcination first, thenpulverized and thereafter molded into a ceramic element 3 aboutelectrodes 4 that extend into grooves 5 on both sides of hole 2 as shownin FIG. 1. Thus the ceramic element 3 may be fixed to the support member1 at four points, with the electrode wires 4 protected by the inorganicadhesive 6. Samples of SnO₂ or of exhaust gas sensors thus prepared willbe designated "A1" to "A6".

EXAMPLE B

Anhydrous SnCl₄ (250 g) is dissolved in 1 liter of water, the solutionis neutralized with 300 ml. of 15N ammonia water, and the reactionmixture is allowed to stand overnight at room temperature to obtain amatured stannic acid sol. With addition of 1 liter of water, thereaction mixture is centrifuged, and the supernatant is discarded. Thestannic acid sol thus obtained contains a substantial quantity of Cl⁻and is thereafter heated to 350° C. in a rotary kiln to prepare astannic acid gel. This step evaporates the excess of ammonia, dehydratesthe sol, and sublimes NH₄ Cl. The gel is then calcined and fashionedinto an exhaust gas sensor in the same manner as described above inExample A. Samples of SnO₂ or of exhaust gas sensors thus prepared willbe designated "B1" to "B6" and "B41" to "B45".

EXAMPLE C1

Sn(CO₂)₂ is heated in an oxygen atmosphere at 600° to 1200° C. for 3hours to obtain SnO₂, which is used for fabricating an exhaust gassensor of the construction shown in FIG. 1. Sn(CO₂) is used as thestarting material to avoid the influence of anion impurities.

EXAMPLE C2

Anhydrous SnCl₄ (250 g) is added to 1 liter of water and neutralizedwith 300 ml of 15N ammonia water, and the reaction mixture is allowed tostand overnight at room temperature. With addition of 1 liter of water,the mixture is centrifuged. This centrifuging procedure is repeateduntil the Cl⁻ concentration of the supernatant is reduced to below thelower limit detectable with silver nitrate test paper. The resultingstannic acid sol is heated in air at 800° to 1100° C. to obtain SnO₂,which is fabricated into an exhaust gas sensor of the construction shownin FIG. 1. Although the aqueous SnCl₄ solution is neutralized with NH₃in the above process, NH₃ may be replaced by a substance, such as (NH₄)₂CO₃, which releases NH₃ on reacting with the SnCl₄ solution.

Table 1 shows the S, d and σ(d)/d values of the typical samplescharacterized by FIGS. 2 to 7.

                  TABLE 1                                                         ______________________________________                                        Surface Areas and Crystal Sizes of Samples                                    Sample No. S (m.sup.2 /g)                                                                              -d (Å)                                                                            σ(d)/-d                                ______________________________________                                        .sup. A1.sup.1                                                                           27             150                                                 A2         12             370                                                 A3         6.0            700    0.35                                         A4         2.9           1300                                                 A5         1.4           2200                                                 A6         0.6           5500                                                 .sup. B1.sup.2                                                                           20             220    0.45                                         B2         13.5           350                                                 B3         6.0            650    0.35                                         B4         2.3           1600                                                 B5         1.2           2700    0.25                                         B6         0.5           6500    0.15                                          B41       0.84          5000    0.15                                          B42       2.7           1100    0.4                                           B4*       2.3           1600    0.35                                          B43       1.7           1900    0.3                                           B44       1.3           2700    0.25                                          B45       0.72          6500    0.15                                         .sup. C1.sup.3                                                                           2.8           1300    0.1                                          .sup. C2.sup.4                                                                           3.4           1100    0.1                                          ______________________________________                                         *The data for the same sample is listed again for the purpose of              description.                                                                  .sup.1 A stannic acid sol prepared by reacting metal Sn with nitric acid      is used as the starting material for the group a (A1 to A6).                  .sup.2 A stannic acid sol containing C1.sup.-  ions is used as the            starting material for the group B (B1 to B6, B41 to B45). For the samples     B1 to B6, the primary calcination temperature is 600° C., and the      secondary calcination temperatur e is 800 to 1450° C. For the          samples B41 to B45, the primary calcination temperature is 400 to             900° C., and the secondary calcination temperature is 1200°     C.                                                                            .sup.3 Prepared by thermally decomposing Sn(CO.sub.2) at 950° C.       .sup.4 Prepared by thermally decomposing a stannic acid sol, free from        C1.sup.-, at 1000° C.                                             

EFFECT OF PULVERIZATION

Samples were pulverized wet in a ball mill for 24 hours. Table 2,showing the results, reveals that with the samples within the scope ofthe present invention, secondary particles only were broken, i.e. thecrystals per se remaining unbroken. Electron photomicrographs of some ofthe pulverized samples indicate that the top portions of primarycrystals are broken to produce fine crystals, which nevertheless exertno substantial influence on d.

                  TABLE 2                                                         ______________________________________                                        Effect of Pulverization                                                                  Before pulverization                                                                        After Pulverization                                  Sample No. -d (Å)    -d (Å)                                           ______________________________________                                        A3         700           700                                                  B2         350           350                                                  B3         650           650                                                  B5         2700          2700                                                 B6         6500          6000                                                  B43       1900          1900                                                 ______________________________________                                    

HEAT RESISTANCE

Samples were heated at 900° C. for 72 hours in combustion productatmospheres derived from air/fuel ratios where λ=1.1 or λ=0.9 where λ isthe stochimetric point as hereinabove described. Table 3 shows theresulting variations in d, indicating that greater variations occurredin d at λ=0.9, i.e. reducing atmosphere, than at λ=1.1, i.e. oxidizingatmosphere. The heat resistance of the samples is not alwaysproportional to the final calcination temperature. The samples A6 and B6for which full crystal growth has been effected are more susceptible tovariations. The samples B41 and B45, which have been subjected to thesame final calcination as the sample B4, are also prone to variations.The foregoing is summarized in Table 3 below.

                  TABLE 3                                                         ______________________________________                                        Variations In d Due To Heating                                                -d (Å)                                                                    Sample  λ = 0.9   λ = 1.1                                       No.     Before test                                                                             After test Before Test                                                                           After Test                               ______________________________________                                        A2       370       500        370     370                                     A3       700       700        700     700                                     A5      1300      1300       1300    1300                                     A6      5500      7000       5500    6000                                     B1       220       400        220     250                                     B2       350       600        350     350                                     B3       650       700        650     650                                     B4      1600      1600       1600    1600                                     B5      2700      2700       2700    2700                                     B6      6500      8000       6500    7000                                      B41    5000      7000       5000    5500                                      B45    6500      8000       6500     700                                     ______________________________________                                    

This data suggests that the problems associated with the durability ofexhaust gas sensors is attributable to hot reducing atmospheres and thatthe problem can be overcome by the use of SnO₂ wherein crystal growth issuitably inhibited rather than by the use of SnO₂ wherein relativelyuninhibited crystal growth has been permitted.

RESISTIVITY, TEMPERATURE COEFFICIENT OF RESISTANCE AND SENSITIVITY

FIGS. 9 to 11 show the resistivities, temperature coefficients ofresistance and sensitivities of various samples. In these drawings, thesolid line represents resistivities at λ=1.1, and the broken line thoseat λ=0.9. The distance between the solid line and the broken linerepresents the sensitivity to the change of atmospheres. The resultsachieved with the samples B1 to B4 (FIG. 9) show that the samples arenot much different in resistivity at λ=0.9 but differ in resistivity atλ=1.1. At λ=1.1, the resistivity of the exhaust gas sensor increaseswith the growth of crystals, and this tendency becomes more pronouncedat higher temperatures. If the ratio between the resistivities at λ=1.1and λ=0.9 is considered to be sensitivity, then sensitivity improveswith crystal growth, especially at higher temperatures.

FIG. 10 shows the results achieved with an exhaust gas sensor (B5) withsuitable growth of crystals and exhaust gas sensors (B41, B45 and B6)with excessive crystal growth. In samples with excessive crystal growth,markedly increased resistivities result at λ=0.9, and reducedsensitivity. Such sensors also exhibit increased resistivities also atλ=1.1 and are difficult to use at low temperatures.

FIG. 11 shows the results achieved with exhaust gas sensors (B4 and A4)having enlarged in crystal size distribution and an exhaust sensors (C1and C2) diminished in distribution. Although the samples are not muchdifferent in surface area S and mean crystal size d, the samples C1 andC2 are considerably lower in sensitivity than the samples B4 and A4 at400° C. This indicates that an increased distribution of crystal sizesleads to an enhanced sensitivity at low temperatures.

When the samples of group a (A1 to A6) were made to resemble one anotherin S and d, the results achieved were similar to those attained with thesamples of group B (B1 to B6) and therefore will not be described.

RESPONSIVENESS

FIGS. 12 and 13 show the responsiveness of exhaust gas sensors to thechange of λ from 1.1 to 0.9. The sensors used were B4 and C1 as typicalexamples embodying the invention, B2 as an example with insufficientcrystal growth, and B41 as an example with excessive crystal growth. At400° C. (FIG. 12), B4 is similar to B2 in responsiveness, whereas C1 isdelayed in response. The samples B4 and C1 differ in crystal sizedistribution but are otherwise similar. It is seen that a widenedcrystal size distribution results in improved responsiveness at lowtemperatures. No data is shown for B41 because it was difficult todetermine the responsiveness thereof at 400° C. due to its highresistivity.

At 900° C. (FIG. 13), B4 and C1 are rapid and similar in response,whereas B2 and B41 are slower in response. The sensor B2 requires morethan 1 minute before starting to respond to a change to the oxidizingatmosphere. When subjected to repeated cycles of λ=1.1 and λ=0.9, theresistivity at λ=1.1 decreases to about 1/4 the level (at the 32nd cyclestarting at 124th minute). On the other hand, B41 is merely slow in themode of response, starting to respond immediately, and suffers lesserhysteresis when repeatedly subjected to the cycle of λ=1.1 to λ=0.9. Theabove phenomena are not limited to specific samples only; other samplesare also subject to such phenomena depending on their S's and meancrystal size d.

VARIATIONS IN RESISTIVITY DUE TO EXPOSURE TO REDUCING ATMOSPHERES

Table 4 shows variations in the resistivity of exhaust gas sensorsresulting from exposure to an atmosphere at 900° C. and λ=0.9.

When exposed to the above atmosphere for 1 minute (Test (1)), thesamples B1, B2 have great difficulty in restoring the initialresistivity even if λ is restored to 1.1. This corresponds to the lagpreceding the start of response and shown in FIG. 13. The samples B6,B41 and B45, although slow in restoring their resistivity, do notinvolve such a lag.

When exposed to the reducing atmosphere for 10 minutes (Test (2)), thesamples B1 and B2 with insufficient crystal growth and those B6, B41 andB45 with excessive crystal growth all undergo hysteresis in resistivity,exhibiting reduced resistivities even one hour after the change of λ to1.1.

When exposed to the reducing atmosphere for 12 hours (Test (3)), thesamples B1, B2, B6 B41 and B45 fail to restore the initial resistivityat λ=1.1. Even 3 days after the return of λ to 1.1, the sample B2, forexample, restores the resistivity only to 70% of the initial level. Thissample is inherently low in sensitivity at high temperatures and istherefore influenced greatly by the variation of resistivity.

                  TABLE 4                                                         ______________________________________                                        Variations in Resistivity Due To                                              Exposure To Reducing Atmosphere*.sup.1                                                  Test (2)                                                            Sample Test (1) In 10  In 1 Test (3)                                          No.    In 1 min.                                                                              min    hr.  In 10 min                                                                            In 1 day                                                                             In 3 days                           ______________________________________                                        A3     0.8      0.96   0.99 0.98   0.97   1.02                                A4      0.98    0.99   1.01 0.97   0.97   1.01                                A5     0.8      0.95   1.01 0.98   0.99   0.99                                B1°                                                                           0.1      0.2    0.3  0.1    0.5    0.6                                 B2°                                                                           0.1      0.3    0.45 0.15   0.6    0.7                                 B3     0.7      0.9    0.99 0.9    0.97   0.99                                B4      0.97    1.02   0.99 0.98   1.01   0.99                                B5     0.8      0.95   1.01 0.92   0.98   0.98                                B6°                                                                           0.4      0.55   0.6  0.3    0.7    0.9                                 B41°                                                                           0.44    0.6    0.7  0.3    0.7    0.9                                 B42     0.98    0.98   1.01 0.97   0.99   1.03                                B4*.sup.2                                                                             0.97    1.02   0.99 0.98   1.01   0.99                                B43    0.9      1.01   0.99 0.95   0.98   0.97                                B44    0.8      0.96   1.00 0.92   0.97   0.98                                B45°                                                                          0.4      0.6    0.7  0.3    0.7    0.9                                 ______________________________________                                         *.sup.1 The sensor is disposed within an apparatus maintained at              900° C. and λ = 1.1, and the atmosphere is temporarily          changed to λ = 0.9. The result is given in terms of the ratio of       the resistivity of the sample as measure d after the atmosphere is            thereafter returned to λ = 1.1, to the resistivity before the          exposure to the reducing atmosphere. The duration of exposure to the          reducing atmosphere is 1 minute for Test (1), 10 minutes for Test (2) and     12 hours for T est (3).                                                       .sup.*2 The data for the same sample is given for the purpose of              description.                                                                  ° The mark "°" indicates a comparison example.             

Although it appears that the samples B6, B41 and B45 generally return totheir initial resistivity, these samples have a tendency to exhibit anincreased resistivity when heated at λ=1.1 at 900° C., so that theapparent data will not be indicative of the restoration of theresistivity.

CHARACTERISTICS IN HOT OXIDIZING ATMOSPHERE DETERMINED WITH LAPSE OFTIME

FIG. 14 shows variations in the resistivity of sensors when they areexposed to an atmosphere at λ=1.1 at 900° C.

With the sensors B4 and B5, the resistivity decreases during the firstday and thereafter levels off, whereas with the sensors B41 and B6, theresistivity steadily increases without any tendency to level off. Inview of the fact that the sensors B41 and B6 originally have a highresistivity, these sensors do not appear useful even if the resistivitycould be stabilized to a constant level with a lapse of time.

OPERATION OF THE EXHAUST GAS SENSOR

Sensors constructed in accordance with this invention may be used toregulate the air/fuel ratio in combustion type devices such ascombustion engines, space heaters, furnaces, and the like. For example,the sensor may be placed in the exhaust pipe of an internal combustionengine wherein it is heated to operating temperatures between 400° and900° C. The composition of exhaust gases passing through the exhaustpipe is primarily a function of the air/fuel ratio. The electricalresistance of the sensor changes in response to changes in the exhaustgas composition causing the magnitude of a control signal from thesensor to vary. A suitable electromechanical means is provided to adjustthe air/fuel ratio in accordance with the magnitude of the controlsignal.

What is claimed is:
 1. A process for producing combustion gas sensorelements, which comprise:heating a composition including at least onemember of the group consisting of tin and tin compounds in anon-reducing atmosphere to produce SnO₂ crystals having a d, wherein dis the mean particle size, of approximately 500-3200 Å and an S, whereinS is the surface area per unit mass, of approximately 1-8 m² /g;pulverizing the SnO₂ into a ceramic element; and, molding the pulverizedSnO₂ into a ceramic element; and providing the ceramic element withmeans for connection to an electric circuit.
 2. The process forproducing combustion gas sensor elements defined in claim 1, furthercomprising:admixing at least one component selected from the groupconsisting of alumina, amorphous silica, platinum and rhodium.
 3. Theprocess for producing combustion gas sensor elements, defined in claim1, wherein:the means for connecting the ceramic element of an electriccurrent comprises a pair of electrodes embedded in the ceramic element.4. The process for producing combustion gas sensor elements as definedin any one of claims 1, 2, or 3, wherein:the heating is conducted in twosteps comprising:a primary calicination of the composition in anon-reducing atmosphere in the presence of substantial quantities ofanion impurities to produce an SnO₂ intermediate containing betweenabout 0.14 and 0.60 mmol/g.SnO₂ of the anion impurities, and a secondarycalcination of the SnO₂ intermediate in a non-reducing atmosphere attemperatures between about 1000° and 1400° C. to produce SnO₂ having anS of about 1 to 8 m² /g, a d of about 500-3200 Å, and a standarddeviation in crystal size distribution of at least 0.2d.
 5. The processfor producing combustion gas sensor elements defined in claim 4,wherein:at least one anion impurity is selected from the groupconsisting of Cl⁻, Br⁻, I⁻, F⁻, NO₃ ⁻ and SO₄ ⁻².
 6. The process forproducing combustion gas sensor elements defined in claim 4, wherein:thecomposition is a stannic acid sol containing at least 0.11mmol/g.Sn(OH)₄ anion impurity.
 7. The process for producing combustiongas sensor elements defined in claim 6, wherein:the stannic acid sol isprepared by dissolving tin metal in nitric acid.
 8. The process forproducing combustion gas sensor elements defined in claim 6, wherein:thestannic acid sol is prepared by hydrolyzing an aqueous solutioncontaining Sn⁺⁴ ions with at least one member selected from the groupconsisting of ammonia and compounds thereof.
 9. The process forproducing combustion gas sensor elements defined in claim 6, wherein:theprimary calcination step includes dehydrating the stannic acid sol to anacid gel and further heating the gel in a non-reducing atmosphere at450°-800° C. thereby producing SnO₂.
 10. The process for producingcombustion gas sensor elements defined in claim 5, wherein:thecomposition contains Sn(CO₂)₂ and the primary calcination is conductedat 800°-1050° C.